Nitride semiconductor device and method for manufacturing same

ABSTRACT

According to one embodiment, a nitride semiconductor device includes a first semiconductor, a second semiconductor layer, a third semiconductor layer, a fourth semiconductor layer, a first electrode, a second electrode and a third electrode. The first, second and fourth semiconductor layers include a nitride semiconductor. The second semiconductor layer is provided on the first semiconductor layer, has a band gap not less than that of the first semiconductor layer. The third semiconductor layer is provided on the second semiconductor layer. The third semiconductor layer is GaN. The fourth semiconductor layer is provided on the third semiconductor layer to have an interspace on a part of the third semiconductor layer, has a band gap not less than that of the second semiconductor layer. The first electrode is provided on a portion of the third semiconductor layer. The fourth semiconductor layer is not provided on the portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2011-014280, filed on Jan. 26,2011; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a nitride semiconductorand a method for manufacturing the same.

BACKGROUND

Semiconductor devices using a nitride semiconductor can improve thetrade-off relationship between the breakdown voltage and the ONresistance to achieve a decreased ON resistance and an increasedbreakdown voltage because of the excellent material characteristicsthereof. Examples of the structure of such a nitride semiconductordevice include a field effect transistor using a hetero-structure ofAlGaN and GaN. When it is attempted to achieve normally OFF operation inthe structure, a recess gate structure is used in which an AlGaN layerbelow a gate electrode is made thinner than the other portions byetching. Further improvement is required in order to obtain stablecharacteristics in such a nitride semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating the configuration of a nitridesemiconductor device according to a first embodiment;

FIG. 2A to FIG. 4B are schematic cross-sectional views illustrating amethod for manufacturing the nitride semiconductor device according tothe first embodiment;

FIG. 5 is a schematic cross-sectional view illustrating a nitridesemiconductor device according to a second embodiment;

FIG. 6 is a schematic cross-sectional view illustrating theconfiguration of a nitride semiconductor device according to a thirdembodiment;

FIG. 7A to FIG. 10 are schematic cross-sectional views illustrating amethod for manufacturing the nitride semiconductor device according tothe third embodiment;

FIG. 11 is a schematic cross-sectional view illustrating another exampleof the nitride semiconductor device according to the third embodiment;and

FIG. 12 is a schematic cross-sectional view illustrating a nitridesemiconductor device according to a fourth embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a nitride semiconductor deviceincludes a first semiconductor, a second semiconductor layer, a thirdsemiconductor layer, a fourth semiconductor layer, a first electrode, asecond electrode and a third electrode. The first semiconductor layerincludes a nitride semiconductor. The second semiconductor layer isprovided on the first semiconductor layer, has a band gap not less thana band gap of the first semiconductor layer, and includes a nitridesemiconductor. The third semiconductor layer is provided on the secondsemiconductor layer, the third semiconductor layer being GaN. The fourthsemiconductor layer is provided on the third semiconductor layer to havean interspace on a part of the third semiconductor layer, has a band gapnot less than a band gap of the second semiconductor layer, and includesa nitride semiconductor. The first electrode is provided on a portion ofthe third semiconductor layer, the fourth semiconductor layer being notprovided on the portion. The second electrode is provided on one side ofthe first electrode on the fourth semiconductor layer and joined to thefourth semiconductor layer by ohmic junction. The third electrode isprovided on another side of the first electrode on the fourthsemiconductor layer and joined to the fourth semiconductor layer byohmic junction.

In general, according to one embodiment, a method is disclosed formanufacturing a nitride semiconductor device. The method can includeforming a first semiconductor layer including a nitride semiconductor, asecond semiconductor layer provided on the first semiconductor layer,and a third semiconductor layer provided on the second semiconductorlayer on a support substrate. The second semiconductor layer has a bandgap not less than a band gap of the first semiconductor layer, andincludes a nitride semiconductor. The third semiconductor layer is GaN.The method can include forming a fourth semiconductor layer provided onthe third semiconductor layer to have an interspace on a part of thethird semiconductor layer. The fourth semiconductor layer has a band gapnot less than a band gap of the second semiconductor layer, and includesa nitride semiconductor. The method can include forming a firstelectrode on a portion of the third semiconductor layer. The fourthsemiconductor layer is not formed on the portion. In addition, themethod can include forming a second electrode on one side of the firstelectrode on the fourth semiconductor layer and a third electrode onanother side of the first electrode on the fourth semiconductor layer.

Various embodiments will be described hereinafter with reference to theaccompanying drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thickness and width of portions, the proportional coefficients ofsizes among portions, etc., are not necessarily the same as the actualvalues thereof. Further, the dimensions and proportional coefficientsmay be illustrated differently among drawings, even for identicalportions.

In the specification of the application and the drawings, componentssimilar to those described in regard to a drawing thereinabove aremarked with the same reference numerals, and a detailed description isomitted as appropriate.

First Embodiment

FIG. 1 is a schematic view illustrating the configuration of a nitridesemiconductor device according to a first embodiment.

As shown in FIG. 1, a nitride semiconductor device 110 according to thefirst embodiment includes a first semiconductor layer 3, a secondsemiconductor layer 4, a third semiconductor layer 5, and a fourthsemiconductor layer 6. Furthermore, the nitride semiconductor device 110includes a first electrode 10, a second electrode 8, and a thirdelectrode 9. In the nitride semiconductor device 110, the firstsemiconductor layer 3 is formed via a buffer layer 2 formed on a supportsubstrate 1. Here, for convenience of description, the direction fromthe first semiconductor layer 3 toward the second semiconductor layer 4is referred to as upward (the upper side), the opposite direction isreferred to as downward (the lower side).

The first semiconductor layer 3 includes a nitride semiconductor. Thesecond semiconductor layer 4 is provided on the first semiconductorlayer 3. The second semiconductor layer 4 has a band gap not less thanthe band gap of the first semiconductor layer 3 and includes a nitridesemiconductor. The third semiconductor layer 5 is provided on the secondsemiconductor layer 4. The third semiconductor layer 5 is a nitridesemiconductor and includes a smaller amount of aluminum than the secondsemiconductor layer 4. The fourth semiconductor layer 6 is provided onthe third semiconductor layer 5 so as to have an interspace on a part ofthe third semiconductor layer 5. The fourth semiconductor layer 6 has aband gap not less than the band gap of the second semiconductor layer 4and includes a nitride semiconductor.

The nitride semiconductor device 110 shown in FIG. 1 is a normally OFFfield effect transistor.

In the specification, “nitride semiconductor” includes allsemiconductors expressed by the chemical formula ofB_(α)In_(β)Al_(γ)Ga_(1-α-β-γ)N (0≦α≦1,0≦β≦1,0≦γ≦1,0≦α+β+γ≦1) in whichcomposition ratios α, β, and γ are changed in the respective ranges.Furthermore, in the chemical formula mentioned above, those furtherincluding a group V element other than N (nitrogen), those furtherincluding various elements added in order to control various propertiessuch as the conduction type, and those further including variouselements unintendedly included are also included in the “nitridesemiconductor.”

In the embodiment, GaN and AlGaN, which are group III-V nitridesemiconductors, are used as examples of the nitride semiconductor.

An undoped Al_(x)Ga_(1-x)N (0≦X≦1) is used for the first semiconductorlayer 3. In the embodiment, the first semiconductor layer 3 is GaN. Thefirst semiconductor layer 3 functions as a channel layer. Here,“undoped” refers to a state where intended impurity doping is notperformed.

An undoped or an n-type Al_(Y)Ga_(1-y)N (0≦Y≦1, X 5 Y) is used for thesecond semiconductor layer 4. As an example, in the embodiment, thesecond semiconductor layer 4 is AlGaN with an Al content of 25 percent(%). GaN, for example, which is a nitride semiconductor not includinghighly reactive Al, is used for the third semiconductor layer 5. GaN ischemically stable and less oxidizable than AlGaN, and is a material thatbonds to other impurities less easily. The third semiconductor layer 5is formed on the second semiconductor layer 4 that is AlGaN. The secondsemiconductor layer 4 and the third semiconductor layer 5 have athickness not causing two-dimensional electron gas 7 at the interfacebetween the third semiconductor layer 5 and the second semiconductorlayer 4.

An undoped or an n-type Al_(Z)Ga_(1-Z)N (0≦Z≦1, X≦Z) is used as thefourth semiconductor layer 6. As an example, in the embodiment, thefourth semiconductor layer 6 is AlGaN with an Al content of 25%. In theembodiment, the fourth semiconductor layer 6 has the same band gap asthe second semiconductor layer 4.

The fourth semiconductor layer 6 is provided on the third semiconductorlayer 5 so as to have an interspace on a part of the third semiconductorlayer 5. Specifically, the fourth semiconductor layer 6 is provided in aregion other than the region where the first electrode 10 describedlater will be formed. The fourth semiconductor layer 6 has a thicknessenough to generate the two-dimensional electron gas 7 at the interfacebetween the third semiconductor layer 5 and the second semiconductorlayer 4.

The first electrode 10 is provided above a portion of the thirdsemiconductor layer 5 where the fourth semiconductor layer 6 is notprovided. The first electrode 10 is formed via an insulating film 11provided on the third semiconductor layer 5. In the embodiment, thefirst electrode 10 is a gate electrode. The gate electrode is a MIS(metal insulator semiconductor) gate electrode. The structure of thegate electrode is not limited to MIS but may be a Schottky gateelectrode in which the third semiconductor layer 5 and the firstelectrode 10 are joined by Schottky junction.

The second electrode 8 is provided on one side of the first electrode 10on the fourth semiconductor layer 6. The second electrode 8 is joined tothe fourth semiconductor layer 6 by ohmic junction. In the embodiment,the second electrode 8 is a source electrode.

The third electrode 9 is provided on the other side (the opposite sideto the side where the second electrode 8 is provided) of the firstelectrode 10 on the fourth semiconductor layer 6. The third electrode 9is joined to the fourth semiconductor layer 6 by ohmic junction. In theembodiment, the third electrode 9 is a drain electrode.

The nitride semiconductor device 110 thus configured has a recessstructure in which the semiconductor layer immediately below the firstelectrode 10 that is the gate electrode is thinner than thesemiconductor layer immediately below the second electrode 8 and thethird electrode 9 that are the source electrode and the drain electrode.In the recess structure, since the second semiconductor layer 4 of AlGaNand the third semiconductor layer 5 of GaN existing below the firstelectrode 10 have a small thickness, the two-dimensional electron gas 7is not generated at the interface between the first semiconductor layer3 that is the channel layer and the second semiconductor layer 4. Thus,the nitride semiconductor device 110 forms a normally OFF field effecttransistor.

Furthermore, the portions other than the recess structure have ahetero-structure of AlGaN and GaN, and can achieve a low resistance.

This can realize a normally OFF nitride semiconductor device with a lowresistance and a high breakdown voltage.

FIG. 2A to FIG. 4B are schematic cross-sectional views illustrating amethod for manufacturing the nitride semiconductor device according tothe first embodiment.

First, as shown in FIG. 2A, the buffer layer 2 of, e.g., AlN is formedon the support substrate 1 of, e.g., SiC. Next, the first semiconductorlayer 3 of, e.g., GaN, the second semiconductor layer 4 of, e.g., AlGaNthereon, and the third semiconductor layer 5 of, e.g., GaN thereon areformed on the buffer layer 2. The first semiconductor layer 3, thesecond semiconductor layer 4, and the third semiconductor layer 5 arecontinuously formed by epitaxial growth. In other words, the workpieceis not taken out of the furnace of epitaxial growth until the firstsemiconductor layer 3 to the third semiconductor layer 5 are formed.

Next, as shown in FIG. 2B, the workpiece is taken out of the furnace ofepitaxial growth in a state where components up to the thirdsemiconductor layer 5 are formed, and an insulating film 13 is formed onthe third semiconductor layer 5. The insulating film 13 is, for example,SiO₂. In the insulating film 13, the portion where the fourthsemiconductor layer 6 described later will be formed are removed, andonly the portion where the first electrode 10 will be formed is left.

Next, as shown in FIG. 3A, the insulating film 13 is used as a maskmember to form the fourth semiconductor layer 6 of, e.g., AlGaN on thethird semiconductor layer 5. The fourth semiconductor layer 6 is formedby epitaxial growth in a portion other than the insulating film 13.After the fourth semiconductor layer 6 is formed, the insulating film 13is removed as shown in FIG. 3B.

Next, as shown in FIG. 4A, the insulating film 11 is formed so as tocover the fourth semiconductor layer 6 and the exposed portion of thethird semiconductor layer 5. After that, as shown in FIG. 4B, parts ofthe insulating film 11 on the fourth semiconductor layer 6 are removed,and the first electrode 10 that is the gate electrode is formed in aportion where the fourth semiconductor layer 6 is not formed. Then, thesecond electrode 8 that is the source electrode and the third electrode9 that is the drain electrode are formed on portions where the parts ofthe insulating film 11 on the fourth semiconductor layer 6 are removed(the exposed portions of the fourth semiconductor layer 6). Thiscompletes the nitride semiconductor device 110.

In the nitride semiconductor device 110 according to the embodiment, thethickness of the second semiconductor layer (AlGaN) 4 that determinesthe threshold of the field effect transistor and the thickness of thethird semiconductor layer (GaN) 5 are accurately determined by theconditions of epitaxial growth. Therefore, the threshold of the fieldeffect transistor can be made uniform as compared to the case where arecess structure is formed by the etching of semiconductor layers.

Furthermore, since the fourth semiconductor layer (AlGaN) 6 formed in aportion other than the region where the first electrode 10 that is thegate electrode is formed is formed thick by regrowth, thetwo-dimensional electron gas 7 is generated at the interface between thefirst semiconductor layer (GaN) 3 and the second semiconductor layer(AlGaN) 4 corresponding to this portion, and a low resistance can beachieved.

In the nitride semiconductor device 110 according to the embodiment, thefirst semiconductor layer 3 to the third semiconductor layer 5 growepitaxially in a continuous manner without being taken out of thefurnace. Hence, it is after GaN is formed at the uppermost layer (theuppermost surface) of the stacked body that the stacked body ofsemiconductor layers is taken out of the furnace. GaN is more chemicallystable and less oxidizable than AlGaN. Therefore, in the case where thestacked body is formed and then temporarily taken out of the furnace,and the insulating film 13 that is a mask member is formed, after whichthe workpiece is again put into the furnace to epitaxially grow thefourth semiconductor layer 6, the fourth semiconductor layer 6 that isAlGaN can be grown in a crystalline manner on the GaN which is lesssusceptible to oxidization.

Here, since Al has high reactivity, if the workpiece is taken out of thefurnace in a state where the uppermost layer of the stacked body isAlGaN, a thin oxide film is easily formed on the surface of the AlGaNlayer, or bonding of the AlGaN with an impurity easily occurs.Consequently, a crystal defect or a trap level resulting from a crystaldefect is easily formed in the semiconductor layer formed on the AlGaN.If such a trap level exists, a carrier may be trapped or released tocause a characteristic variation. Furthermore, a conduction path may beformed via a trap to cause an increase in the leakage current or adecrease in the breakdown voltage.

In the nitride semiconductor device 110 according to the embodiment,since the workpiece is taken out of the furnace in a state where theuppermost layer of the stacked body is hardly oxidizable GaN, it isdifficult for a crystal defect to be generated in the layer that isformed on the GaN by epitaxial growth performed again. Such a trap levelis less easily formed, and the threshold variation of the field effecttransistor can be suppressed. Therefore, a nitride semiconductor device110 with a limited risk of characteristic abnormality and characteristicvariation can be obtained.

Second Embodiment

FIG. 5 is a schematic cross-sectional view illustrating a nitridesemiconductor device according to a second embodiment.

In regard to a nitride semiconductor device 112 according to the secondembodiment, a description is given mainly about the difference with thenitride semiconductor device 110 according to the first embodiment.

The nitride semiconductor device 112 includes a fifth semiconductorlayer 12 between the third semiconductor layer 5 and the fourthsemiconductor layer 6.

The fifth semiconductor layer 12 includes an n-type nitridesemiconductor. In the embodiment, AlGaN doped to an n type is used as anexample. In the case where the fourth semiconductor layer 6 is doped toan n type, the doping concentration of the fifth semiconductor layer 12is higher than the doping concentration of the fourth semiconductorlayer 6.

The doping concentration of the fifth semiconductor layer 12 ispreferably not less than 1×10¹⁸ cm⁻³, for example.

The fifth semiconductor layer 12 is formed on the third semiconductorlayer 5 by epitaxial growth. The fourth semiconductor layer 6 iscontinuously formed on the fifth semiconductor layer 12 by epitaxialgrowth.

Providing the fifth semiconductor layer 12 allows the trap level at theregrowth interface to be compensated. That is, also in the case wherethe fourth semiconductor layer 6 is grown in a crystalline manner on thethird semiconductor layer 5 as in the case of the nitride semiconductordevice 110 according to the first embodiment, there is a possibilitythat trap levels will be formed in a small number at the regrowthinterface. By providing the fifth semiconductor layer 12 that is ahighly doped layer between the third semiconductor layer 5 and thefourth semiconductor layer 6, the trap level at the regrowth interfacecan be compensated.

That is, the fifth semiconductor layer 12 facilitates the re-release ofcarriers trapped. Furthermore, since the fifth semiconductor layer 12prevents an electric field from being applied to the regrowth interface,carrier trapping occurs less easily. Therefore, a nitride semiconductordevice 112 with a limited risk of characteristic variation can beobtained.

If the fifth semiconductor layer 12 that is a highly doped layer has alarge thickness, a conduction path is produced via the highly dopedlayer, which leads to an increase in the leakage current and a decreasein the threshold voltage. Therefore, the thickness of the fifthsemiconductor layer 12 needs to be set so that the fifth semiconductorlayer 12 may be depleted and include no free carrier. Specifically, thethickness of the fifth semiconductor layer 12 is preferably fivenanometers (nm) or less.

Third embodiment

FIG. 6 is a schematic cross-sectional view illustrating theconfiguration of a nitride semiconductor device according to a thirdembodiment.

In regard to a nitride semiconductor device 113 according to the thirdembodiment, a description is given mainly about the difference with thenitride semiconductor device 110 according to the first embodiment.

In the nitride semiconductor device 113 according to the thirdembodiment, the third semiconductor layer 5 is provided on part of thesecond semiconductor layer 4.

The fourth semiconductor layer 6 is provided on both sides of the thirdsemiconductor layer 5 on the second semiconductor layer 4.

In the nitride semiconductor device 113 according to the thirdembodiment, the third semiconductor layer 5 is not provided between thesecond semiconductor layer 4 and the fourth semiconductor layer 6.Therefore, the ohmic resistance between the second electrode 8 that isthe source electrode and the third electrode 9 that is the drainelectrode, and the fourth semiconductor layer 6 and the secondsemiconductor layer 4 can be reduced as compared to the case where thethird semiconductor layer 5 is interposed.

FIG. 7A to FIG. 10 are schematic cross-sectional views illustrating amethod for manufacturing the nitride semiconductor device according tothe third embodiment.

First, as shown in FIG. 7A, the buffer layer 2 of, e.g., AlN is formedon the support substrate 1 of, e.g., SiC. Next, the first semiconductorlayer 3 of, e.g., GaN, the second semiconductor layer 4 of, e.g., AlGaNthereon, and the third semiconductor layer 5 of, e.g., GaN thereon areformed on the buffer layer 2. The first semiconductor layer 3, thesecond semiconductor layer 4, and the third semiconductor layer 5 arecontinuously formed by epitaxial growth. In other words, the workpieceis not taken out of the furnace of epitaxial growth until the firstsemiconductor layer 3 to the third semiconductor layer 5 are formed.

Next, as shown in FIG. 7B, the workpiece is taken out of the furnace ofepitaxial growth in a state where components up to the thirdsemiconductor layer 5 are formed, and the insulating film 13 is formedon the third semiconductor layer 5. The insulating film 13 is, forexample, SiO₂. In the insulating film 13, the portion where the fourthsemiconductor layer 6 described later will be formed is removed and onlythe portion where the first electrode 10 will be formed is left.

Next, as shown in FIG. 8A, the insulating film 13 is used as a maskmember to selectively etch the third semiconductor layer 5. Theselective etching of the third semiconductor layer 5 is performed by,for example, heat treatment in a mixed atmosphere of hydrogen andammonia. This removes the portions other than the portion masked withthe insulating film 13 of the third semiconductor layer 5.

Next, as shown in FIG. 8B, the insulating film 13 is used as a maskmember to form the fourth semiconductor layer 6 of, e.g., AlGaN on thethird semiconductor layer 4. The fourth semiconductor layer 6 is formedby epitaxial growth on the exposed portion other than the insulatingfilm 13 of the second semiconductor layer 4. After the fourthsemiconductor layer 6 is formed, the insulating film 13 is removed asshown in FIG. 9A.

Next, as shown in FIG. 9B, the insulating film 11 is formed so as tocover the fourth semiconductor layer 6 and the exposed portion of thethird semiconductor layer 5. After that, as shown in FIG. 10, parts ofthe insulating film 11 on the fourth semiconductor layer 6 are removed,and the first electrode 10 that is the gate electrode is formed in aportion where the fourth semiconductor layer 6 is not formed. Then, thesecond electrode 8 that is the source electrode and the third electrode9 that is the drain electrode are formed on portions where the parts ofthe insulating film 11 on the fourth semiconductor layer 6 are removed(the exposed portions of the fourth semiconductor layer 6). Thereby, thenitride semiconductor device 113 is completed.

Also in the nitride semiconductor device 113 according to the thirdembodiment, similarly to the nitride semiconductor device 110 accordingto the first embodiment, since the second semiconductor layer 4 of AlGaNand the third semiconductor layer 5 of GaN below the first electrode 10have a small thickness, the two-dimensional electron gas 7 is notgenerated at the interface between the first semiconductor layer 3 thatis the channel layer and the second semiconductor layer 4. Thus, thenitride semiconductor device 113 forms a normally OFF field effecttransistor. The thickness of the second semiconductor layer (AlGaN) 4that determines the threshold of the field effect transistor and thethickness of the third semiconductor layer (GaN) 5 are accuratelydetermined by the conditions of epitaxial growth. Therefore, thethreshold of the field effect transistor can be made uniform as comparedto the case where a recess structure is formed by the etching ofsemiconductor layers. Furthermore, since the fourth semiconductor layer(AlGaN) 6 formed in a portion other than the region where the firstelectrode 10 that is the gate electrode is formed is formed thick byregrowth, the two-dimensional electron gas 7 is generated at theinterface between the first semiconductor layer (GaN) 3 and the secondsemiconductor layer (AlGaN) 4 corresponding to this portion, and a lowresistance can be achieved.

Moreover, in the nitride semiconductor device 113 according to the thirdembodiment, since the third semiconductor layer (GaN) 5 other than theregion immediately below the first electrode 10 that is the gateelectrode is removed, the increase in the ohmic resistance due to thepresence of GaN can be suppressed.

In the method for manufacturing the nitride semiconductor device 113,before the epitaxial growth of the fourth semiconductor layer 6 shown inFIG. 8B is performed, thermal cleaning may be performed on the secondsemiconductor layer (AlGaN) 4 shown in FIG. 8A. The thermal cleaning isa treatment in which the support substrate 1 with the secondsemiconductor layer 4 exposed is put into a furnace for performingepitaxial growth and heated to a prescribed temperature to remove oxidesand the like at the surface of the second semiconductor layer 4.

In the case where thermal cleaning is performed on the secondsemiconductor layer 4, the surface of the second semiconductor layer 4is slightly removed.

FIG. 11 is a schematic cross-sectional view illustrating another nitridesemiconductor device according to the third embodiment.

In a nitride semiconductor device 113A shown in FIG. 11, thermalcleaning is performed on the second semiconductor layer 4.

Therefore, the thickness of the second semiconductor layer 4 immediatelybelow the fourth semiconductor layer 6 is thinner than the thickness ofthe second semiconductor layer 4 immediately below the first electrode10.

Oxides and the like at the surface of the second semiconductor layer 4are removed by the thermal cleaning of the second semiconductor layer 4,and the fourth semiconductor layer 6 is epitaxially grown in this state.Therefore, during the growth of the fourth semiconductor layer 6, thegeneration of a crystal defect can be suppressed and a nitridesemiconductor device 113A with a limited risk of characteristicabnormality and characteristic variation can be obtained.

Fourth Embodiment

FIG. 12 is a schematic cross-sectional view illustrating a nitridesemiconductor device according to a fourth embodiment.

In regard to a nitride semiconductor device 114 according to the fourthembodiment, a description is given mainly about the difference with thenitride semiconductor device 113 according to the third embodiment.

The nitride semiconductor device 114 includes the fifth semiconductorlayer 12 between the second semiconductor layer 4 and the fourthsemiconductor layer 6.

The fifth semiconductor layer 12 is similar to that of the nitridesemiconductor device 112 according to the second embodiment. That is,the fifth semiconductor layer 12 includes an n-type nitridesemiconductor. The fifth semiconductor layer 12 preferably has athickness of, for example, 5 nm or less.

Providing the fifth semiconductor layer 12 allows the trap level at theregrowth interface to be corrected. That is, in the case where thefourth semiconductor layer 6 is grown in a crystalline manner on thesecond semiconductor layer 4 as in the case of the nitride semiconductordevice 113 according to the third embodiment, there is a possibilitythat trap levels will be formed in a small number at the regrowthinterface. Hence, by providing the fifth semiconductor layer 12 that isa highly doped layer between the second semiconductor layer 4 and thefourth semiconductor layer 6, the trap level at the regrowth interfacecan be corrected.

That is, the fifth semiconductor layer 12 facilitates the re-release ofcarriers trapped. Furthermore, since the fifth semiconductor layer 12prevents an electric field from being applied to the regrowth interface,carrier trapping occurs less easily. Therefore, a nitride semiconductordevice 114 with a limited risk of characteristic variation can beobtained.

Embodiments are described above, but the invention is not limited tothese examples. For example, although a description is given using acombination of AlGaN and GaN as semiconductor layers in the embodiments,also combinations such as GaN and InGaN, and AlN and AlGaN may be used.

Furthermore, the structure and thickness of the buffer layer 2 arearbitrary to the extent that a GaN layer or the like with good crystalquality can be formed thereon. In addition, also the support substrate 1is arbitrary to the extent that a GaN layer or the like with goodcrystal quality can be formed. Moreover, although examples of the fieldeffect transistor are described in the embodiments, the embodiments canbe easily applied to other elements such as Schottky barrier diodes.

As described above, by the nitride semiconductor device and the methodfor manufacturing the same according to the embodiments, in the casewhere a recess structure is formed by the regrowth of a semiconductorlayer, the formation of a defect at the regrowth interface can besuppressed, and a normally OFF nitride semiconductor device with alimited risk of variation in the threshold can be obtained. Therefore, anitride semiconductor device with stable characteristics can beprovided.

Hereinabove, embodiments and variations thereof are described. However,the invention is not limited to these examples. For example, one skilledin the art may appropriately make additions, removals, and designchanges of components to the embodiments or the variations describedabove, and may appropriately combine features of the embodiments; suchvariations also are included in the scope of the invention to the extentthat the spirit of the invention is included.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

1-7. (canceled)
 8. A nitride semiconductor device comprising: a firstsemiconductor layer including a nitride semiconductor; a secondsemiconductor layer provided on the first semiconductor layer, having aband gap not less than a band gap of the first semiconductor layer, andincluding a nitride semiconductor; a third semiconductor layer providedon a part of the second semiconductor layer, the third semiconductorlayer being GaN; fourth semiconductor layers provided on both sides ofthe third semiconductor layer on the second semiconductor layer, havinga band gap not less than a band gap of the second semiconductor layer,and including a nitride semiconductor; a first electrode provided abovea portion of the third semiconductor layer, the fourth semiconductorlayers being not provided on the portion; a second electrode provided onone side of the first electrode on the fourth semiconductor layers andconnected to the fourth semiconductor layers by ohmic junction; and athird electrode provided on another side of the first electrode on thefourth semiconductor layers and connected to the fourth semiconductorlayers by ohmic junction.
 9. The device according to claim 8, whereinthe first semiconductor layer includes Al_(X)Ga_(1-X)N (0≦X≦1), thesecond semiconductor layer includes Al_(Y)Ga_(1-Y)N (0≦Y≦1, X≦Y), andthe fourth semiconductor layer includes Al_(Z)Ga_(1-Z)N (0≦Z≦1, X≦Z).10. The device according to claim 8, further comprising a fifthsemiconductor layer provided between the second semiconductor layer andthe fourth semiconductor layer, and including an n-type nitridesemiconductor.
 11. The device according to claim 10, wherein the fifthsemiconductor layer includes Al_(m)Ga_(1-m)N (0≦m≦1).
 12. The deviceaccording to claim 8, further comprising an insulating layer providedbetween the first electrode and the third semiconductor layer.
 13. Thedevice according to claim 8, wherein the first electrode is connected tothe third semiconductor layer by Schottky junction.
 14. The deviceaccording to claim 8, wherein the first semiconductor layer includes achannel in a normally OFF transistor.
 15. A method for manufacturing anitride semiconductor device comprising: forming a first semiconductorlayer including a nitride semiconductor, a second semiconductor layerprovided on the first semiconductor layer, and a third semiconductorlayer provided on the second semiconductor layer on a support substrate,the second semiconductor layer having a band gap not less than a bandgap of the first semiconductor layer, and including a nitridesemiconductor, the third semiconductor layer being GaN; forming a fourthsemiconductor layer provided on the third semiconductor layer to have aninterspace on a part of the third semiconductor layer, having a band gapnot less than a band gap of the second semiconductor layer, andincluding a nitride semiconductor; and forming a first electrode above aportion of the third semiconductor layer, the fourth semiconductor layerbeing not formed on the portion, and forming a second electrode on oneside of the first electrode on the fourth semiconductor layer and athird electrode on another side of the first electrode on the fourthsemiconductor layer.
 16. A method for manufacturing a nitridesemiconductor device comprising: forming a first semiconductor layerincluding a nitride semiconductor, a second semiconductor layer providedon the first semiconductor layer, and a third semiconductor layerprovided on a part of the second semiconductor layer on a supportsubstrate, the second semiconductor layer having a band gap not lessthan a band gap of the first semiconductor layer, and including anitride semiconductor, the third semiconductor layer being GaN; formingfourth semiconductor layers having a band gap not less than a band gapof the second semiconductor layer and including a nitride semiconductoron both sides of the third semiconductor layer on the secondsemiconductor layer; and forming a first electrode above a portion ofthe third semiconductor layer, the fourth semiconductor layers being notformed on the portion, and forming a second electrode on one side of thefirst electrode on the fourth semiconductor layers and a third electrodeon another side of the first electrode on the fourth semiconductorlayers.